The magic bullet - a biomolecule which recognizes a specific target - was proposed in the 1950's. However, the use of selective antibodies to kill tumors has had limited success partially due to the immunogencity of murine antibodies made with hybridoma technology. This limitation has been overcome by obtaining human antibodies from libraries displayed on phage. Data collected at the Stanford Synchrotron Radiation Laboratory, Beamline 9-1 permitted structure determination on extremely small antibody crystals. Consequently, we have determined the first struture of a human antibody from a phage library. Structural analysis has facilitated coupling of toxic moieties to the antibody without altering antibody binding. This antibody is in pre-clinical trials supported by the National Cancer Institute.